That police footage, and a lengthy effort to keep it from the public, came to define the tenure of Mayor Rahm Emanuel and led to a painful, prolonged reckoning for this city and its leaders. On Tuesday, about 22 hours before the scheduled start of jury selection in Officer Van Dyke’s murder trial, Mr. Emanuel announced that he would not seek re-election in February.

Laquan’s death changed Chicago, in ways obvious and still to be determined. It laid bare decades of distrust over Chicago police officers’ treatment of black residents and over City Hall’s lack of transparency. It led to policy changes and promises of a revamped Police Department. And it pushed Mr. Emanuel, a Democrat who built a reputation as a master tactician during stints in Congress and on two White House staffs, into a political bind that he endured but never overcame.

After the dash camera video was released in late 2015, Mr. Emanuel resisted protesters’ calls to resign and rebutted their claims of a top-to-bottom cover-up. But he also fired the police superintendent and promised, tearfully at times, a wholesale rethinking of how officers interact with black residents and how they use their weapons.

Nearly three years later, on the eve of Officer Van Dyke’s trial, many Chicagoans said they were dissatisfied with the mayor’s progress. Trust in the police remains low, the homicide rate remains high, and new police shootings continue to bring tense protests.

“This city’s in a world of hurt right now, and this trial and this election I think are watershed moments in this city’s history,” said Garry F. McCarthy, the police superintendent whom Mr. Emanuel fired and who is now a candidate to replace him as mayor. “Anybody can see that we’re taking on water in all compartments.”

Mr. Emanuel, who declined to be interviewed for this article, surprised many in Chicago when he called a news conference with less than an hour’s notice on Tuesday morning and said he would not seek another term. He did not explicitly mention the McDonald case, and Adam Collins, a spokesman for the mayor, said the trial was not a factor in the decision.

At the same time, only a few miles from City Hall, Officer Van Dyke’s final pretrial hearing was underway at the county courthouse. During a break, a sheriff’s deputy casually informed lawyers and courtroom spectators of the mayor’s announcement; some reporters scrambled to leave to cover the story.

A large field of challengers had already lined up to run against Mr. Emanuel, and many had made gun violence, policing and the McDonald case central to their campaigns.

As the images of Laquan crumpling to the ground set off marches through Chicago’s downtown, protesters accused the mayor of keeping the video under wraps to help in a difficult re-election fight. “Sixteen shots and a cover-up” became a rallying cry.

“Rahm Emanuel’s will was forced,” said the Rev. Ira Acree, a critic of the mayor who leads a West Side church and who said trust in the police continues to recede.

An Officer on Trial

As Officer Van Dyke’s trial begins, Chicago is watching intently, nervously. Protesters have vowed to gather outside the county courthouse every day. Officer Van Dyke, who is free on bond and on unpaid leave from the Police Department, has pleaded not guilty. He told The Chicago Tribune last week that he behaved as he had been taught.

“I might be looking at the possibility of spending the rest of my life in prison for doing my job as I was trained as a Chicago police officer,” Officer Van Dyke told The Tribune.

Police officers have wide latitude to use deadly force. But prosecutors say Laquan posed no lethal threat and that Officer Van Dyke’s decision to shoot, and keep shooting, was unreasonable. Of at least seven other officers at the scene, none fired a single bullet. Later, many of them provided an account of the shooting that was favorable to Officer Van Dyke but not supported by the video.

That night, two officers trailed the teenager at a distance and asked dispatchers to send an officer with a Taser. More officers arrived. At one point, city officials said, Laquan pounded on the window of a squad car and punctured its front tire with a knife. Still, officers followed at a distance for several blocks as Laquan jogged ahead of their cruisers.

The tenor changed as soon as Officer Van Dyke pulled up. He quickly got out of his cruiser with his gun drawn and started shooting as Laquan moved slightly away from him. Even after the teenager collapsed onto the road, the shots kept coming.

“That video was such a flash point — just watching it felt like you were being injured,” said Lori E. Lightfoot, who at the time was president of the Chicago Police Board, an oversight agency, and who now is running for mayor.

No Chicago police officer has been convicted of murder for an on-duty shooting since 1970, according to The Tribune, and recent high-profile police shooting trials in Minnesota, Missouri, Ohio, Oklahoma and Wisconsin have ended in acquittals or mistrials. There are exceptions, including last week in Texas, where an officer in suburban Dallas was convicted of murder in the shooting death of an unarmed black teenager.

A Tense Summer

Image

A neighborhood in Chicago’s Near West Side.CreditAlyssa Schukar for The New York Times

Faith in the Chicago Police Department remains so low that many people instinctively reject the official version of events.

When a “bait truck” loaded with Nike shoes was parked on the South Side this summer and officers waited to arrest any thieves, residents were appalled by the idea and blamed the police. City officials insisted the project was the idea of a railroad company.

And when officers fatally shot Harith Augustus, an African-American barber, on a busy street in July, it outraged neighbors and led to a tense night of protests. Police officials scrambled to release body camera footage showing that Mr. Augustus had a gun. Many said they remained upset about how officers handled the encounter.

“I don’t have a trust” in the police, said Patricia Summers, a South Side resident. Ms. Summers, who is black, said officers have unfairly profiled members of her family. “Chicago has been so corrupt for so long it would be a force of God in order to change it.”

Chicago, which has roughly even numbers of white, black and Hispanic residents, has long struggled with segregation and economic inequity. Complaints about the police, especially common in black and Hispanic neighborhoods, have been backed up by formal reviews.

But Mr. Johnson, who is black and grew up in Chicago, said he recognized why some remain skeptical. “Let’s face it,” he said, “we’re fighting decades of past history with the relationships between the black community and the Police Department.”

A version of this article appears in print on , on Page A14 of the New York edition with the headline: Chicago Officer’s Trial Is Reckoning for City ‘in a World of Hurt’. Order Reprints | Today’s Paper | Subscribe